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Date: Sun, 28 Jun 2009 04:08:00 -0500
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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To: Zaphod Beeblebrox <zbeeble@gmail.com>
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Cc: freebsd-fs <freebsd-fs@freebsd.org>,
	Rafael Caesar Lenzi <rc_lenzi@yahoo.com.br>
Subject: Re: Adding more disk's on ZFS array
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Zaphod Beeblebrox wrote:
> Not entirely true. For a RAID 0 stripe, yes, you can just add a disk. Be
> clear, however, that existing data is not striped to that disk, but the new
> disk is used for new data.
>   

It's probably best not to think of a ZFS pool as a RAID 0 but instead as
a set of vdev storage areas.

All of the vdevs are candidates for new data writes, depending on free
space, etc.

> According to
> documentation, each pool should be of the same RAID type.  It doesn't,
> however, specify that each set of RAID 5 disks should have the same number
> of disks in it.  This seems to mean that you could add a set of 3 disks
> (raid 5) to an existing raid 5 array with 5 disks.
>   

A pool is a set of vdevs, and different vdevs may be of a different type
and have different characteristics.  It is perfectly reasonable to
create a pool with a single RAIDZ vdev and later add MIRROR vdevs, or
any other kind of vdev.

I prefer to use MIRRORs as the vdevs since it's easier to control
exposure to various failure modes (power supply, enclosure, controller &
disk firmware, etc).